Phosphatidylinositol 3-kinase is a heterodimeric protein consisting of a catalytic 110 kDa subunit and an (.alpha.-85 kDa subunit that is necessary for docking to growth factor plasma membrane receptors including the insulin receptor and to signalling phosphotyrosine motifs in signalling proteins, e.g. insulin receptor substrate-1 (1,2). In addition to its association with receptors that mediate their effects via tyrosine kinase activity, PI 3-kinases are also implicated in facilitating membrane trafficking processes. The pinocytic activity, membrane ruffling and actin reorganization response observed when KB cells are exposed to insulin have been shown to be due to PI3-kinase activation (3,4). PI3-kinase has been implicated as a key intermediate in the cytoskeletal rearrangements that accompany secretory processes in platelets, basophil and neutrophil cells (5,6). Similarly, the yeast homologue VPS34 has been shown to be involved in vesicle trafficking and protein sorting (7). PI3-kinase is thus an attractive candidate protein as a mediator of insulin action on glucose transport as it could act as a point of convergence of signalling and trafficking processes.
In our attempts to elucidate the genetics of insulin action and glucose metabolism, we have performed mutational analysis on the regulatory p85.alpha. subunit of PI3K which is expressed in skeletal muscle.